2-Component crosslink of end-functionalized polyacrylates

ABSTRACT

Method for increasing the molecular weight of polyacrylates, in which polyacrylates, functionalized at least in part of their chain ends by functional groups X, are reacted with a compound having at least functional groups Y capable of linking reactions with functional groups X.

The invention relates to a process for increasing the molecular weightof polyacrylates and their derivatives, especially for crosslinking.

Among producers of acrylate pressure sensitive adhesives (PSAs) there isa trend toward reducing the proportion of solvent in the productionprocess. This relates in particular to the coating process, since herein general the polymers are coated from a solution with a concentrationof 20 or 30% onto the corresponding carrier material and subsequentlythe solvent is distilled off again in drying tunnels. As a result of theheat introduced, the drying step may additionally be utilized for thethermal crosslinking of the PSA.

If it is then desired to reduce the solvent fraction or to eliminate itcompletely, polyacrylate PSAs can be coated from the melt. This is doneat relatively high temperatures, since otherwise the flow viscositywould be too high and the adhesive would exhibit an extreme resilienceduring the coating operation. One example of a functioning commercialsystem is represented by the UV acResins™ from BASF AG. Here, a low flowviscosity at temperatures of less than 140° C. has been achieved bylowering the average molecular weight to below 300 000 g/mol.Accordingly, these materials are easy to coat from the melt. As a resultof the lowering, however, there is also a deterioration in the technicaladhesive properties, especially the cohesion, of these PSAs. Inprinciple, the cohesion can be raised by UV or EB crosslinking.Nevertheless, the UV acResins™ do not achieve the level of cohesionattained by high molecular mass acrylate PSAs which have been appliedconventionally from solution and crosslinked thermally.

A key problem is the network arc length, since acrylate hotmelt PSAsgenerally have a relatively low molecular weight, possess a relativelylow fraction of interloops, and thus need to be crosslinked to a greaterextent. Although the greater crosslinking does increase the level ofcohesion, the distance between the individual crosslinks becomes smallerand smaller. Consequently, the network is significantly tighter and thePSA then possesses only a low level of viscoelastic properties.

Accordingly, there is a need for a polymer which is easy to coat fromthe melt and is subsequently crosslinked on the carrier material in filmform in a specific way, so that, preferably, a linear polymer with onlya very few crosslinking sites is formed.

Endgroup-functionalized polymers have already been known for a longtime. In U.S. Pat. No. 4,758,626, for example, polyesters were impactmodified using carboxy-terminated polyacrylates. However, no descriptionwas given there of specific endgroup crosslinking.

U.S. Pat. No. 4,699,950 describes thiol-functionalized polymers andblock copolymers. The polymers, however, contain only one functionalgroup, which is subsequently used for polymerization or for otherreaction.

U.S. Pat. No. 5,334,456 describes maleate- or fumarate-functionalizedpolyesters. Subsequent crosslinking takes place in the presence of vinylethers. Here again, polyacrylates are not described.

U.S. Pat. No. 5,888,644 describes a process for preparing releasecoating materials. Its starting point is formed by polyfunctionalacrylates, which are reacted with polysiloxanes. Here again, no definednetwork is formed, so that this process too cannot be transferred toacrylate PSAs.

U.S. Pat. No. 6,111,022 describes poly(meth)acrylonitrile polymersprepared by ATRP. Terminally functionalized polymers can also beprepared by these processes. Advantageous processes for preparingpurposively crosslinked PSAs are not disclosed, however.

In U.S. Pat. No. 6,143,848, terminally functionalized polymers areprepared by a new, controlled polymerization process. The polymerizationprocess employed is an iodine transfer process. However, polymers ofthis type lack great thermal stability, since iodides generally reactwith air and are easily oxidized to iodine. Severe discolorations are aconsequence. This applies in particular to hotmelt processes with hightemperatures.

None of the aforementioned documents indicates a process in whichendgroup-functionalized polyacrylates were mixed with a second componentand reacted with it in a deliberate way in order to construct a linearpolymer chain or a polymer network.

It is an object of the invention to specify a process for building upthe molecular weight of polyacrylates, in particular for crosslinkingthereof, which exhibits the disadvantages of the prior art only to areduced extent, if at all.

Surprisingly, and unforeseeably for the skilled worker, this object isachieved by the process of the invention, as set out in the independentclaim and in the subclaims.

The invention accordingly provides a process for increasing themolecular weight of polyacrylates, polyacrylates functionalized at leaston some of their chain ends by suitable groups X being reacted with atleast one compound containing at least two functional groups Y capableof entering into linking reactions with the functional groups X.

Here and below, the general term polyacrylates should be taken toinclude derivatives thereof and also polymethacrylates and derivativesthereof, additionally referred to as component (a).

In a first very advantageous version of the process the linkingreactions are addition reactions. In a second very advantageous versionof the process the linking takes place by way of substitution reactions.Substitution reactions are taken to mean, with particular advantage,esterification and transesterification reactions.

In a further, likewise very advantageous version of the process thepolyacrylates containing the functional groups X are reacted inaccordance with the invention with a compound containing at least twofunctional groups Y capable of bonding the polyacrylates using thefunctional groups Y. Bond formations of this kind are, for example, thebonding of the polyacrylates at two coordination centers, as a complex;in this sense functional centers as well should be understood asfunctional groups. In the respective compounds the respective functionalgroups X and Y are located at the chain ends of the compounds and aretherefore also referred to below as functional endgroups.

The compounds containing functional groups Y are also referred to belowas linking compounds and are designated component (b).

The polyacrylates functionalized with the groups X very advantageouslyhave an average molecular weight (number average) M_(n) in the rangefrom 2000 g/mol to 1 000 000 g/mol. The process is consequentlyparticularly suitable for the construction or for the crosslinking ofpolyacrylate PSAs.

For the purposes of the process of the invention an increase inmolecular weight is understood in particular to refer to crosslinking,but also to the construction of molecules of higher molecular mass(longer-chain molecules). The process therefore allows compounds of highmolecular mass to be constructed from the lower molecular masscomponents: in one version, which is particularly preferred for theprocess of the invention, the components (that is, the polyacrylatescontaining the functional groups X and the linking compounds containingthe functional groups Y) are linked linearly to one another. Accordinglyit is possible, for example, to construct high molecular massalternating block copolymers from the low molecular mass components,with particular advantage in such a way that the blocks each correspondto one of the monomeric units. However, compounds already in blockcopolymer form can also be linked.

A further particularly preferred version of the process of the inventioncomprises the synthesis of crosslinked structures from the polyacrylatesand the linking compounds. In this case it is very advantageous if atleast one of the two components (polyacrylates and/or linking compounds)possess at least three functional terminal groups. In the sense of theinvention, therefore, it is advantageous if the polyacrylates containingthe functional groups X and/or the linking compounds containing thefunctional groups Y contain at least one and, where appropriate,preferably two or more chain branches, so that there are more than twochain ends.

With further advantage at least one of the two components then containsthree or more functional groups Y.

The network density can be increased further by both components carryingat least three or more terminal functional groups. As the functionalitygoes up, so does the tendency to form networks. This is the case evenwhen the number of functionalities rises for only one of the components.

In order to obtain particular desired linking results it is possible fordi-, tri- and/or polyfunctional polyacrylates to be mixed with di-, tri-and/or polyfunctional linking compounds, and reacted.

The mixing ratio of the polyacrylates and of the linking compounds canbe chosen freely, in accordance with the target property of the linkedpolymer. With advantage the amount of functional groups X correspondsessentially to that of functional groups Y. The molar ratio n_(Y)/n_(X)of the number n_(Y) of the functional groups Y of the linking compoundto the number n_(x) of the functional groups X of the polyacrylates isin each case preferably situated within a magnitude range of between 0.8and 1.2, very preferably between 0.8 and 1.

Reference may be made with particular preference to the followingreactions by way of example in this context; however, the list is notconclusive and is intended merely to illustrate the inventive process byreference to a number of exemplary linking reactions: Group X ofcomponent (a) on polymer side Group Y of component (b) Anhydride-Hydroxy-, Alkoxy-, Mercapto-, Thiol-, Isocyanate-, Amino-, Oxazole-, . .. Acid- Hydroxy- Ester- Amino- Hydroxy- Isocyanate- Acid- Hydroxy-,Alkoxy-, Mercapto-, Thiol-, Anhydride- Isocyanate-, Amino-, Oxazole-, .. . Hydroxy- Acid- Amino- Ester- Isocyanate- Hydroxy- Acid-

The abovementioned reactions proceed by way of addition or substitutionreactions and are generally initiated by means of heat.

In the above exemplary examples, none of the linking sites has beencharacterized in any detail with regard to its chemical nature. Thechemical nature is an automatic consequence—as the skilled worker willbe well aware—of the different co-reactants X and Y. For this reason aprecise description of the linking sites is not given. The major organicreactions can be found, for example, in Advanced Organic Chemistry,Reactions, Mechanisms and Structure, by Jerry March, Wiley Interscience1992.

Polyacrylates Containing Functional Groups X

In a first version of the process of the invention it is possible to uselinear polyacrylates having a terminal functional group X at each of thechain ends. In another preferred version branched, dendritic orstar-shaped polyacrylates are used in the inventive process. Thesepolymers also possess at least two functional endgroups X. In onepreferred version the number of terminal functional groups X correspondsto the number of chain ends or side chain ends or arms of a starpolymer.

Moreover, any linear, branched, dendritic or star-shapedpoly(meth)acrylate may also carry two or more endgroups X at therespective chain end.

In general, a greater number of functional groups X raises thereactivity of the terminally functionalized polyacrylates.

In one preferred version of the process the polyacrylates used arecomposed of at least 50% by weight of acrylic and/or methacrylic acidderivatives of the general structureCH₂═CH(R¹)(COOR²)where R¹═H or CH₃ and R²═H or linear, branched or cyclic, saturated orunsaturated, hydrocarbon radicals having from 1 to 30, in particularfrom 4 to 18, carbon atoms.

It is of advantage for the terminally functionalized polyacrylates tohave a static glass transition temperature of from −100° C. to +25° C.For producing heat-activatable PSAs it is further of advantage to raisethe static glass transition temperature further (preferably up to +175°C.).

For polyacrylate preparation the monomers are chosen such that theresulting polymers can be used for pressure sensitive adhesives at roomtemperature or higher temperatures, particularly such that the resultingpolymers possess pressure sensitive adhesive properties in accordancewith the “Handbook of Pressure Sensitive Adhesive Technology” by DonatasSatas (van Nostrand, New York, 1989). In order to obtain a preferredpolymer glass transition temperature T_(G)≦25° C., in accordance withthe above remarks, the monomers are very preferably selected in such away, and the quantitative composition of the monomer mixtureadvantageously chosen in such a way, that for the polymer the desiredT_(G) is obtained in accordance with the Fox equation (G1) (cf. T. G.Fox, Bull. Am. Phys. Soc. 1 (1956)123). $\begin{matrix}{\frac{1}{T_{G}} = {\sum\limits_{n}\frac{w_{n}}{T_{G,n}}}} & ({G1})\end{matrix}$

In this equation, n represents the serial number of the monomers used,w_(n) denotes the mass fraction of the respective monomer n (in % byweight), and T_(G,n) denotes the respective glass transition temperatureof the homopolymer of the respective monomer n, in K.

Preferably, use is made of acrylates and methacrylates having alkylgroups of 4 to 14 carbon atoms, preferably of 4 to 9 carbon atoms.Specific examples, without wishing to be restricted by this listing,include methyl acrylate, methyl methacrylate, ethyl acrylate, n-butylacrylate, n-butyl methacrylate, n-pentyl acrylate, n-hexyl acrylate,n-heptyl acrylate, n-octyl acrylate, n-octyl methacrylate, n-nonylacrylate, lauryl acrylate, stearyl acrylate, behenyl acrylate, and thebranched isomers thereof, such as isobutyl acrylate, 2-ethyl-hexylacrylate, 2-ethylhexyl methacrylate, isooctyl acrylate, and isooctylmethacrylate, for example.

Further classes of compounds which can be used include monofunctionalacrylates and methacrylates of bridged cycloalkyl alcohols, composed ofat least 6 carbon atoms. The cycloalkyl alcohols may also besubstituted, by C₁₋₄ alkyl, halogen or cyano, for example. Specificexamples include cyclohexyl methacrylates, isobornyl acrylate, isobornylmethacrylates, and 3,5-dimethyladamantyl acrylate.

Furthermore, in an approach which is very advantageous for the process,no use is made of vinyl compounds containing functional groups whichadversely affect the coupling or crosslinking reaction of the functionalgroups X and Y with one another.

Furthermore, it is possible optionally to use monomers from thefollowing groups as what are defined as vinyl monomers: vinyl esters,vinyl ethers, vinyl halides, vinylidene halides, and vinyl compoundscontaining α-positioned aromatic cycles and heterocycles. Here as well,selected monomers which can be used in accordance with the invention maybe mentioned by way of example: vinyl acetate, vinylformamide,vinylpyridine, ethyl vinyl ether, 2-ethylhexyl vinyl ether, butyl vinylether, vinyl chloride, vinylidene chloride, acrylonitrile.

Advantageously, monomers are used which carry polar groups such ascarboxyl, sulfonic and phosphonic acid, hydroxyl, lactam and lactone,N-substituted amide, N-substituted amine, carbamate, epoxy, thiol,ether, alkoxy, cyano or the like.

Examples of moderate basic monomers are N,N-dialkyl-substituted amides,such as N,N-dimethylacrylamide, N,N-dimethylmethylmethacrylamide,N-tert-butylacrylamide, N-vinylpyrrolidone, N-vinyllactam,dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate,diethylaminoethyl methacrylate, diethylaminoethyl acrylate,N-methylol-methacrylamide, N-(butoxymethyl)methacrylamide,N-methylolacrylamide, N-(ethoxy-methyl)acrylamide,N-isopropylacrylamide, this list not being conclusive.

Further suitable, preferred examples of vinyl-functional monomers in thesense of the definition include hydroxyethyl acrylate, hydroxypropylacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate,N-methylolacrylamide, allyl alcohol, styrene, functionalized styrenecompounds, maleic anhydride, itaconic anhydride, itaconic acid, benzoinacrylate, acrylated benzophenone, acrylamide, and glycidyl methacrylate,to name but several.

Moreover, advantageously, photoinitiators having a copolymerizabledouble bond are used. Suitable photoinitiators include Norrish I and IIphotoinitiators. Examples are e.g. benzoin acrylate and an acrylatedbenzophenone from UCB (Ebecryl P 36®). In principle it is possible tocopolymerize any photoinitiators which are known to the skilled workerand which are able to crosslink the polymer by a free-radical mechanismunder UV irradiation. An overview of possible photoinitiators which canbe used and which can be functionalized with a double bond is given inFouassier: “Photoinitiation, Photopolymerization and Photocuring:Fundamentals and Applications”, Hanser-Verlag, Munich 1995. For furtherdetails it is possible to consult Carroy et al. in “Chemistry andTechnology of UV and EB Formulation for Coatings, Inks and Paints”,Oldring (ed.), 1994, SITA, London.

With further preference, comonomers which possess a high static glasstransition temperature are added to the monomers described. Suitablecomponents include aromatic vinyl compounds, such as styrene, in whichcase the aromatic nuclei are preferably composed of C₄ to C₁₈ units andmay also contain heteroatoms. Particularly preferred examples include4-vinylpyridine, N-vinylphthalimide, methylstyrene,3,4-dimethoxystyrene, 4-vinylbenzoic acid, benzyl acrylate, benzylmethacrylate, phenyl acrylate, phenyl methacrylate, t-butylphenylacrylate, t-butylphenyl methacrylate, 4-biphenyl acrylate andmethacrylate, 2-naphthyl acrylate and methacrylate, and mixtures ofthose monomers.

An advantageous possibility is for one or more functional groups to beincorporated which allow radiation-chemical crosslinking of thepolymers, in particular by means of UV irradiation or by irradiationwith rapid electrons. With this objective, monomer units which can beutilized include, in particular, acrylic esters containing anunsaturated alkyl radical having from 3 to 18 carbon atoms andcontaining at least one carbon-carbon double bond. Acrylates modifiedwith double bonds which are suitable with particular advantage includeallyl acrylate and acrylated cinnamates. Besides acrylic monomers,monomers which can be used with great advantage for the polymer blockalso include vinyl compounds having double bonds which are not reactiveduring the (free-radical) polymerization. Particularly preferredexamples of corresponding comonomers are isoprene and/or butadiene, butalso chloroprene.

In a further version of the process, the polyacrylates contain one ormore grafted-on sidechains. Systems of this kind can be prepared both bya graft-from process (polymerizational attachment of a sidechainstarting from an existing polymer backbone) and by a graft-to process(attachment of polymer chains to a polymer backbone by means ofpolymer-analogous reactions). For preparing sidechain polymers of thiskind it is possible in particular to use, as monomers, monomersfunctionalized in such a way as to allow a graft-from process for thegrafting-on of sidechains.

A preferred characteristic of the terminally functionalizedpolyacrylates is that their molecular weight M_(n) is between about 2000 and about 1 000 000 g/mol, preferably between 30 000 and 400 000g/mol, with particular preference between 50 000 and 300 000 g/mol.Preferably the polydispersity of the polymer is less than 3, being thequotient formed from the mass average M_(w) and the number average M_(n)of the molar mass distribution.

In general, the reactivity of low molecular mass, terminallyfunctionalized polyacrylates is higher, and so these are preferablyemployed for the reaction.

In order to prepare the terminally functionalized polyacrylates it ispossible in principle to use all polymerizations which proceed inaccordance with controlled or living mechanisms, thus includingcombinations of different controlled polymerization processes. In thiscontext, without making any claim to completeness, mention may be madeby way of example not only of anionic polymerization but also ATRP, GTRP(Group Transfer Radical Polymerization), nitroxide/TEMPO-controlledpolymerization, or more preferably the RAFT process; in other words,particularly those processes which permit control of the block lengthsand the polymer architecture and which additionally introduce afunctional group into the polyacrylate. Also conceivable areconventional free-radical addition polymerizations, subject to theproviso that the initiator carries at least one functional group whichremains in the polymer after the initiation as well.

In order to prepare the terminally functionalized polyacrylates it ispossible to use different technologies as well. Besides anionicpolymerization the polymers may also be prepared in emulsion or beadpolymerization, solution polymerization, pressure polymerization or elsebulk polymerization processes.

Free-radical addition polymerizations may be conducted in the presenceof an organic solvent or in the presence of water or in mixtures oforganic solvents and/or organic solvents with water, or without solvent.It is preferred to use as little solvent as possible. Depending onconversion and temperature, the polymerization time for free-radicalprocesses is typically between 2 and 72 hours.

In the case of solution polymerization the solvents used are preferablyesters of saturated carboxylic acids (such as ethyl acetate), aliphatichydrocarbons (such as n-hexane, n-heptane or cyclohexane), ketones (suchas acetone or methyl ethyl ketone), special-boiling-point spirit,aromatic solvents such as toluene or xylene, or mixtures ofaforementioned solvents. For the polymerization in aqueous media ormixtures of organic and aqueous solvents, it is preferred to addemulsifiers and stabilizers for the purpose of polymerization.Polymerization initiators used with advantage for the controlledfree-radical polymerizations include customary free-radical-formingcompounds such as peroxides, azo compounds, and peroxosulfates, forexample. Initiator mixtures are also outstandingly suitable.

For the synthesis of the polyacrylates it is possible to usenitroxide-controlled polymerization processes. For the preferreddifunctional polyacrylates it is preferred to use difunctionalinitiators. One example of this are difunctional alkoxyamines (I).

where R^(1*), R^(2*), R^(3*) and R^(4*) may be different, identical orchemically joined to one another and where pairs R^(1*) and R^(2*) andalso R^(3*) and R^(4*) in each case contain at least one group X orpossess a functional group which can be converted into X by chemicalreaction. R^(1*) to R^(4*) are preferably independently of one anotherchosen as:

-   i) halides, such as chlorine, bromine or iodine, for example,-   ii) linear, branched, cyclic and heterocyclic hydrocarbons having    from 1 to 20 carbon atoms, which may be saturated, unsaturated or    aromatic,-   iii) esters —COOR^(5*), alkoxides —OR^(6*) and/or phosphonates    —PO(OR^(7*))₂, where R^(5*), R^(6*) and R^(7*) stand for radicals    from group ii),-   iv) radicals from ii) where additionally at least one hydroxy    function or silyl ether function is present.

For the preparation of the terminally functionalized polyacrylates bynitroxide-controlled polymerization it is also possible to use further,different alkoxyamines. From the basic synthesis design, the middleblock, which following thermal initiation, initiation by thermalradiation or by actinic radiation forms two free radicals, can beadditionally varied or modified further. The skilled worker is aware ofa variety of chemical structures. A precondition is that at least 2 freeradicals are formed which are stabilized by nitroxides which carry atleast one functional group X or a group which is converted into X bymeans of a chemical reaction.

In one favorable procedure, nitroxides of type (II) or (III) are usedfor radical stabilization:

where R^(1#), R^(2#), R^(3#), R^(4#), R^(5#), R^(6#), R^(7#) and R^(8#)independently of one another denote the following compounds or atoms andpreferably at least one of R^(1#) to R^(6#) and R^(7#) and/or R^(8#)carry at least one group X or contain a group which can be convertedinto the desired group X by means of a chemical reaction. R^(1#) toR^(8#) are preferably chosen independently of one another as:

-   i) halides, such as chlorine, bromine or iodine, for example,-   ii) linear, branched, cyclic, and heterocyclic hydrocarbons having    from 1 to 20 carbon atoms, which may be saturated, unsaturated or    aromatic,-   iii) esters —COOR^(9#), alkoxides —OR^(10#) and/or phosphonates    —PO(OR¹¹)₂, where R^(9#), R^(10#) and R^(11#) stand for radicals    from group ii),-   iv) radicals from ii) where additionally at least one hydroxy    function or silyl ether function is present.

Compounds of the above types (II) or (III) may also be attached topolymer chains of any kind (primarily such that at least one of theabovementioned radicals constitutes a polymer chain of this kind) andmay thus be used, for example, as macroradicals or macroregulators forsynthesizing terminally functionalized polymers.

Further general nitroxide-controlled processes for implementingcontrolled free-radical polymerizations are described below. U.S. Pat.No. 4,581,429 A discloses a controlled free-radical polymerizationprocess which uses as its initiator a compound of the formula R′R″N—O—Y,in which Y denotes a free radical species which is able to polymerizeunsaturated monomers. WO 98/13392 Al describes open-chain alkoxyaminecompounds which have a symmetrical substitution pattern. EP 735 052 A1discloses a process for preparing thermoplastic elastomers having narrowmolar mass distributions. WO 96/24620 A1 describes a polymerizationprocess in which very specific radical compounds, such asphosphorus-containing nitroxides based on imidazolidine, are used. WO98/44008 A1 discloses specific nitroxyls based on morpholines,piperazinones and piperazinediones. DE 199 49 352 A1 describesheterocyclic alkoxyamines as regulators in controlled-radicalpolymerizations. Corresponding further developments of the alkoxyaminesor of the corresponding free nitroxides improve the efficiency for thepreparation of polyacrylates (Hawker, contribution to the NationalMeeting of The American Chemical Society, Spring 1997; Husemann,contribution to the IUPAC World Polymer Meeting 1998, Gold Coast).

All of the abovementioned processes can be employed, by introducing oneor more functional groups X on the stabilized nitroxide radical and/oron the polymerization-initiating radical, for preparingendgroup-functionalized polyacrylates.

As a further controlled polymerization method it is possible withadvantage to synthesize the block copolymers using Atom Transfer RadicalPolymerization (ATRP), in which case preferred initiators aremonofunctional or difunctional secondary or tertiary halides and thehalide(s) is (are) abstracted using complexes of Cu, Ni, Fe, Pd, Pt, Ru,Os, Rh, Co, Ir, Ag or Au (EP 0 824 111 A1; EP 826 698 A1; EP 824 110 A1;EP 841 346 A1; EP 850 957 A1). The various possibilities of ATRP arefurther described in U.S. Pat. No. 5,945,491 A, U.S. Pat. No. 5,854,364A and U.S. Pat. No. 5,789,487 A. For preparing the terminallyfunctionalized polyacrylates the corresponding secondary or tertiaryhalide ought already to carry the desired functional group X.Additionally, as a result of the polymerization process, halideendgroups remain in the polymer, and can likewise be converted into thecorresponding functional groups X by means of substitution reactions. Inorder to prepare multiblock or star-shaped structures it is possible toproceed in accordance with the design described in Macromolecules 1999,32, 231-234. There, polyfunctional halides are used for thepolymerization, and must then be reacted in a substitution reaction bypolymer-analogous means to give the desired functional group(s) X.

It may further be of advantage for the process of the invention toprepare endgroup-modified polyacrylates by way of an anionicpolymerization. In this case the reaction medium used preferablyincludes inert solvents, such as aliphatic and cycloaliphatichydrocarbons, for example, or else aromatic hydrocarbons.

The living polymer is generally represented by the structureP_(L)(A)-Me, in which Me is a metal from group I, such as lithium,sodium or potassium, and P_(L)(A) is a growing polymer block of themonomers A. The molar mass of the endgroup-modified poly(meth)acrylateto be prepared is dictated by the ratio of initiator concentration tomonomer concentration.

For the synthesis of the polymer it is preferred to use acrylate andmethacrylate monomers which do not adversely affect, let alone causecomplete termination of, the anionic polymerization process.

For the preparation of blocklike terminally functionalized polyacrylatesit may be of advantage to add monomers for the synthesis of one polymerblock and then, by adding a second monomer, to attach a further polymerblock. Alternatively, suitable difunctional compounds can be linked. Inthis way it is also possible to obtain starblock copolymers(P(B)—P(A))_(n). In these cases, however, the anionic initiator oughtalready to carry the functional group, or the group ought to beobtainable by a subsequent polymer-analogous reaction.

For general anionic polymerizations, examples of suitable polymerizationinitiators include n-propyllithium, n-butyllithium, sec-butyllithium,2-naphthyllithium, cyclohexyllithium, and octyllithium, with this listmaking no claim to completeness. Furthermore, initiators based onsamarium complexes are known for the polymerization of acrylates(Macromolecules, 1995, 28, 7886) and can be used here. With theseinitiators, however, it must be borne in mind that onlymono-endgroup-functional polyacrylates can be obtained by this route, bydiscontinuing the corresponding anionic polymerization. For thepreparation of carboxyl groups this can take place, for example, bymeans of CO₂ with subsequent hydrolysis; for the preparation of hydroxylgroups, for example, by reaction with ethylene oxide and subsequenthydrolysis.

For preparing component (a) by anionic polymerization it is alsopossible to use difunctional initiators, such as1,1,4,4-tetraphenyl-1,4-dilithiobutane or1,1,4,4-tetra-phenyl-1,4-dilithioisobutane, for example. Coinitiatorsmay likewise be employed. Suitable coinitiators include lithium halides,alkali metal alkoxides and alkylaluminum compounds. In one verypreferred version the ligands and coinitiators are chosen such thatacrylate monomers, such as n-butyl acrylate and 2-ethylhexyl acrylate,for example, can be polymerized directly and need not be generated inthe polymer by transesterification with the corresponding alcohol. Inorder to generate the terminal functionalities these anionicpolymerizations are discontinued such that at least one functional groupis generated on the chain end. In the simplest case, for preparinghydroxyl groups, for example, scavenging is carried out with ethyleneoxide, followed by hydrolysis.

Alternatively, for preparing terminally functionalized polyacrylates byanionic polymerization, difunctional initiators can be used whichalready contain at least one functional group in the polymer, which doesnot hinder the anionic polymerization process. Finally, at least oneterminal functional group can be liberated on the poly(meth)acrylate bymeans of a polymer-analogous reaction as well.

A very preferred preparation process conducted is a variant of the RAFTpolymerization (reversible addition-fragmentation chain transferpolymerization). The polymerization process is described in detail, forexample, in the documents WO 98/01478 A1 and WO 99/31144 A1. Suitablewith particular advantage for the preparation of terminallyfunctionalized polyacrylates are trithiocarbonates of the generalstructure R′″—S—C(S)—S—R′″ (Macromolecules 2000, 33, 243-245), by meansof which one or more monomers (acrylates/methacrylates) are polymerizedand portions of the regulator remain as endgroups in the polymer. In thesimplest case, therefore, the trithiocarbonate may consist of onecompound, where R′″ contains a functional group X or a functional groupwhich can be converted into a functional group X by means of a chemicalreaction.

It may further be appropriate to carry out a two-stage polymerization.In a first step, monomers containing at least one functional group X arepolymerized using a trithiocarbonate and then used in a second step topolymerize the (meth)acrylates. The polymerization may take placecontinuously or with discontinuation after the first stage, andsubsequent reinitiation.

The latter method is particularly suitable for preparing terminallyfunctionalized polyacrylates containing two or more functional groups Xat each end. In a version which is preferred for this variant, use ismade, for example, of the trithiocarbonates (IV) and (V) for thepolymerization, with φ ( possibly being a phenyl ring, which isunfunctionalized or may be functionalized by alkyl or aryl substituentslinked directly or via ester or ether bridges, or possibly being a cyanogroup.

In order to promote the polymerization, the control, and the rate ofpolymerization it may be of advantage to use substituted compounds.Examples of possible functionalizations include halogens, hydroxylgroups, epoxy groups, groups containing nitrogen or groups containingsulfur, although this list makes no claim to completeness. Some of thesegroups may in turn be used as functional groups X.

Besides trithiocarbonates, however, it is also possible to use thefollowing structural elements for the controlled polymerization, with Kbeing as defined below:

In order to prepare terminally functionalized polyacrylates with fewgroups X or only one group X, on the other hand, it may be an advantageto use terminally functionalized trithiocarbonates. In one particularlypreferred version, use is made, for example, of trithiocarbonates oftype VIII and IX.

The group X ought not to influence the controlled free-radicalpolymerization. Moreover, the group κ is highly variable, in order toimprove the control of the polymerization or to change thepolymerization rate. K can be C, to C,B alkyl, C₂ to C₁₈ alkenyl, C₂ toC₁₈ alkynyl, in each case linear or branched; aryl, phenyl, benzyl,aliphatic and aromatic heterocycles. Furthermore, K can contain as oneor more groups —NH₂, —NH—R^(VI), —NR^(VI)R^(VII), —NH—C(O) —R^(VI),—NR^(VI)—C(O)—R^(VII), —NH—C(S)—R^(VI), —NR^(VI)—C(S)—R^(VII),

where R^(VI) and R^(VII) can in turn be compounds of the type C₁ to C₁₈alkyl, C₂ to C₁₈ alkenyl, C₂ to C₁₈ alkynyl, in each case linear orbranched; aryl, phenyl, benzyl, aliphatic and aromatic heterocycles, andare independent of one another or the same.

It is, however, also possible to use regulators which carryfunctionalized dithioester groups at the end and which incorporate thesegroups at the end of the polymer. Regulators of this kind can in thesimplest case have the following structure (XII).

In this case, however, the functional group ought not to influence thepolymerization process but should instead remain on the sulfur atoms, sothat this group is incorporated at the end of the polymer chain.Furthermore, the dibenzylic group can be further modified and adapted inorder further to improve the polymerization properties. At this pointmention may be made, merely by way of example, of patents WO 98/01478 A1and WO 99/31144 A1.

In conjunction with the abovementioned controlled free-radicalpolymerizations, initiator systems are preferred which additionallycomprise further free-radical polymerization initiators, especiallythermally decomposing, radical-forming azo or peroxo initiators. Inprinciple, however, any customary initiators that are known foracrylates are suitable. The production of C-centered radicals isdescribed in Houben-Weyl, Methoden der Organischen Chemie, Vol. E19a, p.60 ff. These methods are employed preferentially.

Examples of radical sources are peroxides, hydroperoxides and azocompounds. Some nonexclusive examples of typical radical initiators thatmay be mentioned here include potassium peroxodisulfate, dibenzoylperoxide, cumene hydroperoxide, cyclohexanone peroxide,cyclohexylsulfonyl acetyl peroxide, di-tert-butyl peroxide,azodiisobutyronitrile, diisopropyl percarbonate, tert-butyl peroctoate,and benzpinacol. In one very preferred variant the free-radicalinitiator used is 1,1′-azobis(cyclohexylnitrile) (Vazo 88®, DuPont®) or2,2-azobis(2-methylbutanenitrile) (Vazo 67®, DuPont®). It is alsopossible, furthermore, to use radical sources which release freeradicals only under UV irradiation.

These initiators are also suitable, however, for the other free-radicalpolymerization methods that proceed in accordance with controlled-growthmechanisms.

In the case of the conventional RAFT process, polymerization is normallycarried out only to low degrees of conversion (WO 98/01478 A1) in orderto obtain very narrow molecular weight distributions. As a result of thelow conversions, however, these polymers cannot be used as pressuresensitive adhesives and in particular not as hotmelt PSAs, since thehigh residual monomer fraction adversely affects the technical adhesiveproperties: the residual monomers contaminate the solvent recyclate inthe concentration process and the corresponding self-adhesive tapeswould exhibit very high outgassing. In order to circumvent thisdisadvantage of low conversions, in one particularly preferred versionthe polymerization is initiated a number of times.

In order to produce multiarm, star-shaped or dendritic terminallyfunctionalized poly-acrylates it is likewise possible to employ thepolymerization processes described above. Through modification of theinitiating compound or of the regulator, such compounds are readilyavailable. The following structures show examples of suitable compounds,the compound XIII being a suitable substance for preparing a 12-armpolyacrylate by an ATRP technique, the compound XIV being suitable forpreparing a 6-arm polyacrylate by a RAFT technique, and the compound XVbeing suitable for preparing a 3-arm polyacrylate vianitroxide-controlled reaction.

The abovementioned examples are intended only to be exemplary in nature.Polyacrylates prepared from compound XIII can be converted, for example,by reaction (substitution reaction) of the terminal bromides intosuitable endgroup-functionalized polyacrylates. Polyacrylates preparedfrom compound XIV already possess one functional group X per polymer armas endgroup. The regulator XIV may, however, also carry this functionalgroup at another position on the terminal phenyl rings or else may carrytwo or more functional groups on the terminal phenyl rings.Polyacrylates prepared from compound XV already possess 3 hydroxylgroups per polymer arm as terminal functional groups, which can be usedfor reaction.

The number of arms produced can be controlled by the number of thegroups which are essential for the controlled free-radicalpolymerization. Moreover, it is also possible to exchange, modify ortargetedly substitute functional groups. By means of this measure it ispossible, for example, to increase or lower the control or the rate ofpolymerization. Furthermore, all of the abovementioned polymerizationmethods depict only exemplary methods for preparing terminallyfunctionalized polyacrylates. It is also possible, however, to employall of the methods of controlled polymerization that are familiar to theskilled worker, provided this polymerization method allows theintroduction of functional groups on the polymer, with particularpreference functionalization at the poly(meth)acrylate chain end.

Besides the controlled free-radical methods, further free-radicalpolymerization methods are also suitable for introducing functionalgroups. By way of example mention may be made merely of thiol-regulatedcompounds, in which case the thiols or dithio compounds may likewisecarry functional groups X and thus effect terminal functionalization ofpolyacrylates. Furthermore, functional groups can be introduced into thepolymer as endgroups by means of the initiator. There exist, forexample, commercial azo initiators, which carry free carboxylic acidgroups or hydroxyl groups, which then, likewise by way of thepolymerization, can be installed in the polymer at the ends and utilizedfor the coupling or crosslinking reaction. Another possibility would beto scavenge the free radical polymerization and in that way incorporatea functional group X.

Linking Compounds (Component (b))

Component (b) is used for linking the terminally functionalizedpolyacrylates. Depending on the field of use of the materials beingprepared, these linking compounds possess different properties. For theinventive process it is preferred, however, that the linking compoundspossess at least two terminal functional groups Y for reaction with thepolyacrylates (component (a)) and that these functional groups Y enterinto a chemical reaction with the functional group X of component (a).

As a result of the multiplicity of possible reactions it is possible aslinking compounds to use not only organic compounds but also inorganiccompounds. For certain applications, oligomers or polymers of compoundswith the corresponding functional groups Y are likewise used.

Inorganic or organometallic compounds are used with great preference inorder, for example, to synthesize high molecular mass networks ofcomponent (a) by way of complexations. At their most simple they may bemetal salts, which are reacted with carboxylic acid groups of terminallyfunctionalized polyacrylates. Examples of suitable metal salts arealkali metal halides or alkaline earth metal halides. For the inventiveprocess it is of advantage if these salts are soluble in thepolyacrylate (component (a)). Some examples are LiBr, LiCl, KBr, Kl,magnesium bromide, and calcium bromide. It is also possible to usetransition metal halides, such as zinc chloride or zinc bromide, forexample. Metal chelate complexes are also suitable for coordinatingendgroup-positioned carboxylic acid groups, such as aluminumacetylacetonate, titanium acetylacetonate, titanium tetraisopropoxide,titanium tetrabutoxide, zirconium acetylacetonate, and ironacetylacetonate, for example. These reactions proceed preferentially ina range of above 100° C. For further processing from the melt,reversible reactions may be of great advantage; that is, the metal saltspossess only weak coordinative interactions at high temperatures and,when cooled to room temperature or service temperature, form strongionic or coordinative bonds. As a result of this measure it is easy tomix components (a) and (b) in the melt, and after coating from the meltthey form, for PSA applications, for example, high-viscosity,shearing-resistant pressure sensitive adhesives as they cool on thecarrier.

For the process of the invention it may also be of advantage, however,if component (b) is not activated until after the mixing operation.

In a further very preferred version of the invention, organic compoundshaving at least one carbon atom and two functional groups Y are used forthe process of the invention. Some examples of compounds having onecarbon atom include malonic acid, malodinitrile, and methylenediamine.Examples of linking compounds having 2 carbon atoms and 2 functionalgroups Y are ethylene glycol and succinic acid. Examples of linkingcompounds having 3 carbon atoms for component (b) are glycerol,1,3-propanediol, glutaric acid, and 1,3-diaminopropane. Examples ofcompounds having 4 carbon atoms are 1,4-butanediol, adipic acid,1,2,4-butanetriol, butene-1,4-diol, 1,2,3,4-butanetetracarboxylic acid,1,7-octadiene, diethylenetriamine, and dimethyl adipate. Examples ofcompounds having 5 carbon atoms are 1,1,1-tris(hydroxymethyl)ethane,1,5-pentanediol, and 1,5-diaminopentane. Examples of compounds having 6carbon atoms are triethylene glycol, suberic acid, 1,6-diaminohexane,1,4-diaminocyclohexane, 1,6-diisocyanohexane, N,N′-diallyltartaramide,and 1,4-diazabicyclo[2.2.2]octane. It is also possible to use aromaticlinking compounds containing 6 carbon atoms, such as1,2-phenylenediamine, 1,3-phenylenediamine, 1,4-phenylenediamine,hydroquinone, pyrocatechol, resorcinol, phthalic acid, terephthalicacid, pyromellitic dianhydride, pyromellitic diimide, pyromellitic acid,1,3-dimercaptobenzene, and N,N,N′,N′-tetramethyl-1,4-phenylenediamine.It is also possible advantageously to use compounds having up to 30carbon atoms, which may be aliphatic or aromatic, may containheterocycles and other cyclic structures, which may contain unsaturatedsites, and/or which may contain the following heteroatoms: N, B, O, F,Cl, Br, I, Si, Al, P or S. The stated heteroatoms may also be present inconjunction with one another, e.g., in the form of a phosphate group, asulfonate group or a nitro or nitroso group. Combination with oneanother is also possible, in the form of peroxo linkages, dithiolinkages, and diaza linkages, for example.

For thermal activation it is possible, besides the reaction ofdifunctional and polyfunctional isocyanates, to use preferably blockedisocyanates as well. As component (b) the compounds possess theadvantage that they can be activated for reaction thermally by means ofenergy (heat). An overview of blocked isocyanates is given in U.S. Pat.No. 5,510,443.

Besides the low molecular mass organic compounds it is also possible touse compounds of higher molecular mass (oligomers) or polymers aslinking compounds. As oligomers and polymers use is made advantageously,for example, of polyacrylates, polymethacrylates, polyisobutene,polyethylene, polypropylene, polyvinyl acetate, polyurethane, polyvinylchloride, polystyrene, polycaprolactam, polycaprolactone, polyester,polybenzoates, polysiloxanes, polyethylene/propylene copolymers,polybutadiene, polyisoprene, polybutene, polythiophene, polyacetylene,polyanthracene, polysilanes, polyamides, polycarbonates, polyvinylalcohol, polypropylene oxide, polyethylene oxide, polyphenylene,polychloroprenes, and fluorinated oligomers and polymers. For thelinking reaction, all these oligomers and polymers ought to contain atleast two functional endgroups Y.

Reaction of Components (a) and (b):

Particularly, in the sense of the invention the process is operated insuch a way that for the linking reaction the polyacrylates (component(a)) and the linking compounds (component (b)) are mixed in a reactorwith a mixing apparatus. The coupling reaction may be carried out insolution or without any solvent. Endgroup-functionalized polyacrylatescan therefore be reacted in solution, in which case the solvent used forthe polymerization is preferably employed, or from the melt. Forprocessing from the melt—if component (a) is prepared from solution—thesolvent is removed from the polymer. This may take place most simply byapplying vacuum or, generally, by distillation. Suitable means ofconcentration include concentrating extruders, for example, which areoperated at low shear in order to avoid gelling during the hotmeltprocess. In accordance with the invention, therefore, the solvent ispreferably stripped off in a concentrating extruder under reducedpressure, for which purpose it is possible, for example, to employsingle-screw or twin-screw extruders which preferably distill off thesolvent in different or identical vacuum stages and which possess a feedpreheater. For the hotmelt process component (a) is preferablyconcentrated to more than 99.5%. Different kinds of reactors can be usedfor implementing the linking reaction. In this context the initialviscosity of the individual components and also the viscosity after thelinking reaction are of critical significance. For implementation in ahigh-viscosity medium, extruders and co-kneaders are suitable withparticular preference. Mixing equipment may likewise comprise twin-screwextruders or else ring extruders. In one very advantageous developmentof the invention, the components are combined (compounded) in the samereactor as that in which treatment (reaction) takes place,advantageously in an extruder. This may also be the extruder in whichthe concentration step has already been carried out.

Equipment which has proven very suitable for reaction with highlyviscous components includes a twin-screw extruder (e.g., Werner &Pfleiderer or Welding Engineers) or a co-kneader (e.g., Buss). In thesereactors the optimum reaction conditions are set by means of the lengthof the processing section, the throughput (rotary speed), the kneadingtemperature, and the amount of any catalysts added. The residence timewithin the reactor can be optimized by effectively mixing the reactioncomponents.

In accordance with the flow viscosity of the components used, thereaction proceeds at increased temperatures. For high-viscosity systemsthe chosen temperatures are between 80 and 200° C., in one particularlypreferred range between 110 and 160° C.

For the process of the invention it may likewise be of advantage to varythe molecular weight of the components—especially of the terminallyfunctionalized polyacrylate—in order to improve ease of processing inthe melt. Thus, for example, by reducing the molecular weight it ispossible to lower the flow viscosity and so increase the readiness toreact. Another point is the processibility under shear in an extruder,since relatively low-viscosity and low molecular mass polymers areeasier to process in an extruder and the shearing introduced is thusgreatly reduced.

For the inventive process it is of advantage to choose a continuousprocess regime and/or to operate individual steps of the process in aninline operation. However, the batchwise process regime is alsopossible.

For coupling from solution or for coupling very low-viscositycomponents, other reactors are generally used. Suitable in one verysimple case, for example, are the kind of mixing heads known from thetwo-component polyurethane technology. Here, the two components arebrought together in a chamber, with very rapid and efficient mixing ofthe two components occurring as a result of the flow or pressure. Thereaction begins in the mixing chamber and may also continue, however, inthe following processing operation. Also suitable for reaction fromsolution, for example, are very conventional stirred tanks, which formixing may be provided with different stirrers. Examples of stirrerswhich can be used include anchor stirrers, propeller stirrers, and MIGstirrers, here again the viscosity being a critical factor. Overall, allof the mixing devices familiar to the skilled worker are suitable.

For the linking reaction it may also be an inventive advantage if themixing devices can be heated and if it is therefore possible tointroduce thermal energy which initiates or accelerates the linkingreaction. The energy requirement is dependent on a number of factors,such as the activation energy of the chemical reaction and the molecularweight of the individual components, for example.

A further parameter specific to the process is the reaction time. Thereaction time as well varies depending on the reactivity of theindividual components, the temperature regime, and the viscosity.Accordingly, the reaction time can generally be shortened by means, forexample, of an increase in temperature.

In one specific version of the inventive process, components (a) and (b)are only mixed and are reacted at a later point in time in the process.One example is the processing of components (a) and (b) in an injectionmolding process, where shaping is followed by curing to retain theshape.

For the process of the invention it is of advantage if the conversion ofthe linking reactions is as quantitative as possible. Nevertheless,especially in the case of the linking of high molecular mass components,the reactivity is often fairly low, so that only significantly lowerconversions can be realized. Instead, in some cases the possibility ofside reactions exists. For the process of the invention, at least onelinking reaction between a functional group X and Y ought to proceedsuccessfully. For the reaction of low molecular mass components it is ofparticular advantage for the inventive process if a relatively highconversion of more than 50% is achieved. In one particularly preferredversion, conversions of greater than 75%, based in each case on thereactive groups, are achieved.

For the preparation of a pressure sensitive adhesive by the processdescribed above it may be advantageous to blend components (a) or (b)with tackifier resins before or after the linking reaction. In principleit is possible to use all resins which are soluble in the correspondingpolymers. Suitable tackifier resins include, among others, rosin and itsderivatives (rosin esters, including rosin derivatives stabilized bydisproportionation or hydrogenation, for example), polyterpene resins,terpene-phenolic resins, alkylphenol resins, aliphatic, aromatic andaliphatic-aromatic hydrocarbon resins, to name but a few. Chosenprimarily are those resins which are preferably compatible withcomponent (a). The weight fraction of the resins is typically up to 40%by weight, more preferably up to 30% by weight. Furthermore, it is alsopossible optionally to add plasticizers, fillers (e.g., fibers, carbonblack, zinc oxide, titanium dioxide, chalk, solid or hollow glass beads,microbeads made of other materials, silica, silicates), nucleators,blowing agents, compounding agents and/or aging inhibitors, in the formfor example of primary and secondary antioxidants or in the form oflight stabilizers.

In one preferred version of the invention the linking reaction ofcomponents (a) and (b) is followed by further crosslinking of theprepared polymer with actinic radiation. Additional crosslinkingoperations are generally particularly useful if the polymers prepared bythe abovementioned processes have been linked linearly. Furthermore,however, it may be of advantage to carry out additional crosslinking ofpolymers already prepared by the process of the invention andcrosslinked, and to achieve particular product properties. This appliesin particular to coatings applications, since high degrees ofcrosslinking are necessary for the cure.

For optional crosslinking with UV light, UV-absorbing photoinitiatorsare optionally added to component (a) and/or (b) before or after thelinking reaction. Useful photoinitiators which are very good to useinclude benzoin ethers, such as benzoin methyl ether and benzoinisopropyl ether, for example, substituted acetophenones, such as2,2-diethoxy-acetophenone (available as Irgacure 651® from Ciba Geigy®),2,2-dimethoxy-2-phenyl-1-phenylethanone, dimethoxyhydroxyacetophenone,substituted α-ketols, such as 2-methoxy-2-hydroxypropiophenone, forexample, aromatic sulfonyl chlorides, such as 2-naphthylsulfonylchloride, for example, and photoactive oximes, such as 1-phenyl-1,2-propanedione 2-(O-ethoxycarbonyl)oxime, for example.

The abovementioned photoinitiators and others which can be used,including those of the Norrish I or Norrish II type, may contain thefollowing radicals: benzophenone, aceto-phenone, benzil, benzoin,hydroxyalkylphenone, phenyl cyclohexyl ketone, anthraquinone,trimethylbenzoylphosphine oxide, methylthiophenyl morpholine ketone,aminoketone, azobenzoin, thioxanthone, hexaarylbisimidazole, triazine,or fluorenone, it being possible for each of these radicals additionallyto be substituted by one or more halogen atoms and/or one or morealkyloxy groups and/or one or more amino groups or hydroxyl groups. Arepresentative overview is given by Fouassier: “Photoinitiation,Photopolymerization and Photocuring: Fundamentals and Applications”,Hanser-Verlag, Munich 1995. For further details, it is possible toconsult Carroy et al. in “Chemistry and Technology of UV and EBFormulation for Coatings, Inks and Paints”, Oldring (Ed.), 1994, SITA,London.

A further option in principle is to crosslink polymers, after thelinking reaction, with electron beams. Typical irradiation equipmentwhich may be used includes linear cathode systems, scanner systems, andsegmented cathode systems, where electron beam accelerators areconcerned. A detailed description of the state of the art, and the mostimportant process parameters, can be found in Skelhorne, Electron BeamProcessing, in Chemistry and Technology of UV and EB formulation forCoatings, Inks and Paints, Vol. 1, 1991, SITA, London. The typicalacceleration voltages are situated in the range between 50 kV and 500kV, preferably 80 kV and 300 kV. The scatter doses employed rangebetween 5 to 150 kGy, in particular between 20 and 100 kGy.

For optional crosslinking with actinic radiation, difunctional orpolyfunctional vinyl compounds are optionally added to component (a)and/or (b) before or after the linking reaction: in one preferredversion, difunctional or polyfunctional methacrylates, in a verypreferred version, difunctional or polyfunctional acrylates.

The polymers of the invention may be used with preference for producingpressure sensitive adhesive tapes. Depending on polymer composition,however, these polymers may also be used for film applications, asrelease coating materials, or as pressure sensitive adhesives. Highlyhalogenated polymers could also be used, however, as flame retardants,for example. Moreover, it is also possible to use the polymers preparedby the process of the invention as heat-activatable PSAs. For thisutility the polymer ought to possess a glass transition temperature ofmore than 25° C. For the polymers with a close polymer network,applications in the coatings field are also possible. Polymers having ahigh glass transition temperature, prepared by the inventive process,may likewise be employed as thermoplastics. Given an appropriate choiceof component (b), electrically conducting polymers are also possible.For example, it would also be possible to prepare polymers which, undercurrent, emit light.

Test Methods

A. Shear Stability Times

The test took place in accordance with PSTC-7. A 50 μm thick pressuresensitive adhesive layer is applied to a 25 μm thick PET film. A stripof this sample 1.3 cm wide is bonded to a polished steel plate over alength of 2 cm, by rolling over it back and forth three times using a 2kg roller. The plates are equilibrated for 30 minutes under testconditions (temperature and humidity) but without loading. Then the testweight is hung on, exerting a shearing stress parallel to the bondsurface, and the time taken for the bond to fail is measured. If aholding time of 10 000 minutes is reached, the test is discontinuedbefore the adhesive bond fails.

B. Bond Strength

The testing of the peel adhesion (bond strength) took place inaccordance with PSTC-1. A 50 μm thick pressure sensitive adhesive layeris applied to a 25 μm thick PET film. A strip of this sample 2 cm wideis bonded to a steel plate by rolling back and forth over it three timesusing a 2 kg roller. The steel plate is clamped in and the self-adhesivestrip is pulled off from its free end at a peel angle of 180° using atensile testing machine.

C. Gel Permeation Chromatography GPC

The average molecular weight M_(w) and the polydispersity PD weredetermined by the company Polymer Standards Service of Mainz, Germany.The eluent used was THF containing 0.1% by volume trifluoroacetic acid.Measurement was made at 25° C. The precolumn used was PSS-SDV, 5μ, 10³Å, ID 8.0 mm×50 mm. Separation was carried out using the columnsPSS-SDV, 5μ, 10³ and also 10⁵ and 10⁶ each of ID 8.0 mm×300 mm. Thesample concentration was 4 g/l and the flow rate 1.0 ml per minute.Measurement was made against PMMA standards.

D. Gel Fraction

The carefully dried, solvent-free adhesive samples are welded into apouch of polyethylene nonwoven (Tyvek web). The gel index, i.e., thetoluene-insoluble weight fraction of the polymer, is determined from thedifference in the sample weights before and after extraction withtoluene.

Production of Test Specimens

Preparation of a RAFT Regulator:

The regulator bis-2,2′-phenylethyl trithiocarbonate (VIII) was preparedstarting from 2-phenylethyl bromide using carbon disulfide and sodiumhydroxide in accordance with the set of instructions in Synth. Comm.,1988, 18 (13), 1531.

Yield: 72%. ¹H-NMR (CDCl₃), δ: 7.20-7.40 ppm (m, 10 H); 3.81 ppm (m, 1H); 3.71 ppm (m, 1 H); 1.59 ppm (d, 3 H); 1.53 ppm (d, 3 H).

Preparation of Nitroxides:

(a) Preparation of the Difunctional Alkoxyamine (XVII):

The procedure was carried out in analogy to the experimentalinstructions from Journal of American Chemical Society, 1999, 121(16),3904. Starting materials used were 1,4-divinylbenzene and nitroxide(XVII).

(b) Preparation of the Nitroxide (XVIII):

The procedure followed was analogous to the experimental instructionsfrom Journal of Polymer Science, Polymer Chemistry, 2000, (38), 4749.

Monomers:

The monomers for the nitroxide-controlled polymerizations were purifiedby distillation beforehand and stored under nitrogen. Hydroxyethylacrylate was likewise purified by distillation and then stored under anargon atmosphere at −20° C.

Commercial Raw Materials Used: Trade name Structure Manufacturer Vazo67 ® 2,2′-Azobis(2-methylbutanenitrile) DuPont Vazo 64 ®2,2′-Azobis(isobutyronitrile) DuPont Starburst ® Dendrimer with 16 aminogroups Sigma-Aldrich (PAMAM) (generation 2) and an M_(w) of about 3 256g/mol Esacure Oligomeric polyfunctional photoinitiator Lamberti KIP150 ®

Amine-Functionalized UV Photoinitiator:

3-[4-(Dimethylamino)phenyl]-1-[4-(2-hydroxyethoxy)Phenyl]-2-propen-1-one(XIX):

A mixture of 15 g of 2-bromoethanol, 16.3 g of p-hydroxyacetophenone and5.3 g of sodium hydroxide in 100 ml of dimethylformamide (DMF) washeated at 150° C. for 15 hours. The mixture was then poured into waterand the product was extracted with dichloromethane. Subsequent vacuumdistillation gave 11.4 g of a white solid(4-(2-hydroxyethoxy)acetophenone).

In a second reaction, a mixture of 8.3g of p-dimethylaminobenzaldehyde,10.0g of 4-(2-hydroxyethoxy)acetophenone and 2.5 g of sodium hydroxidein 100 ml of methanol was heated at reflux for 10 hours. The reactionmixture was then cooled using an ice bath and filtered and the solidisolated by filtration was washed with cold methanol. The product wasthen dried in a vacuum drying cabinet at 40° C. and 10 torr. 10.2 g ofwhite solid were isolated. The melting point was 128° C. (cf. U.S. Pat.No. 4,565,769, m.p.: 127-128.5° C.)

Preparation of a functionalized RAFT regulator, S,S-di(benzyl-4-phenylacetic acid)trithiocarbonate (XX)

5 g of 4-bromomethylphenylacetic acid were slowly deprotonated in 20 mlof an aqueous solution using 5 ml of 33% strength sodium hydroxidesolution, with stirring. In parallel, a three-necked flask was chargedwith 10 ml of carbon disulfide, 10 ml of 33% strength sodium hydroxidesolution and also 0.92 g of n-butylammonium hydrogen sulfate. Throughvigorous stirring, the emulsion changed in color from brown toblood-red. The solution prepared above was then slowly added inportions. The color of the emulsion changed to yellow. Followingcomplete reaction (reaction time about 48 hours), the two phases wereseparated from one another in a separating funnel. The aqueous phase isextracted three times with carbon disulfide and then a number of timeswith special-boiling-point spirit. The organic phases are then combinedand extracted by shaking a number of times with 0.1 N hydrochloric acid.The organic phase is dried over magnesium sulfate and then the solventis removed on a rotary evaporator. The residue is an oily, reddishyellow liquid. The yield was 90%.

IR: 1 065 cm⁻¹ (C=S) ¹H-NMR (CDCl₃): δ=7.34 (8 H, m); 4.65 (4 H, s);3.61 (4 H, s) ¹³C-NMR (CDCl₃): δ=223 (SC(S)S); 178.2 (COOH)

Implementation of the Hotmelt Process in a Recording Extruder

The compounding of the hotmelt PSAs was carried out using the recordingextruder Rheomix 610p from Haake. The Rheocord RC 300p drive unit wasavailable. The apparatus was controlled using the PolyLab Systemsoftware. The extruder was charged in each case with 52 g of pureacrylate hotmelt PSA (˜80% filling level). The experiments wereconducted with a kneading temperature of 140° C., a rotary speed of 40rpm, and a kneading time of 15 minutes.

Reference 1

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 40 g of acrylic acid, 0.4 g of bis-2,2′-phenylethyltrithiocarbonate and 160 g of DMF. The batch was rendered inert withnitrogen gas while being stirred with an anchor stirrer at roomtemperature for 1 hour. It was then heated to an internal temperature of58° C. using an oil bath, after which 0.2 g of Vazo 64™ (from DuPont)(2,2′-azobis(isobutyronitrile)) in solution in 5 g of DMF was added.After a polymerization time of 24 hours, the batch was cooled to roomtemperature and DMF was distilled off on a rotary evaporator. Analysisby GPC (test C, PMMA standards) gave a molecular weight M_(n) of 2 820g/mol and Mw of 7 540 g/mol.

Subsequently this oligomeric polyacrylate, 300 g of 2-ethylhexylacrylate and 60 g of methyl acrylate were dissolved in 150 g ofacetone/n-butanol (7:3) and the solution was rendered inert usingnitrogen gas for 1 hour and then heated to an internal temperature of58° C. again. At this temperature, 0.2 g of Vazo 64™(DuPont)(2,2′-azobis-(isobutyronitrile)) in solution in 5g of acetone was added.The polymerization was conducted at a constant external temperature of70° C. Following a reaction time of 6 hours, the batch was diluted with80 g of acetone. After a reaction time of 24 hours, a further 0.2 g ofVazo 64™ (DuPont) (2,2′-azobis(isobutyronitrile)) in solution in 5 g ofacetone was added. After 30 hours the batch was diluted with 50 g ofacetone. The polymerization was terminated by cooling to roomtemperature after a reaction time of 48 hours. Analysis by GPC (test C,PMMA standards) gave a molecular weight Mn of 166 000 g/mol and M_(w) of421 000 g/mol.

Thereafter the solvent was removed in a drying cabinet at 60° C. under avacuum of 10 torr and a 30% strength solution in acetone was prepared.This solution was then applied to a primed PET film 23 μm thick. Afterdrying for 10 minutes at 120° C. in a drying cabinet, the applicationrate of the polymer was 50 g/m². The technical adhesive properties weretested by carrying out test methods A and B.

EXAMPLE 1

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 40 g of acrylic acid, 0.4 g of bis-2,2′-phenylethyltrithiocarbonate and 160 g of DMF. The batch was rendered inert withnitrogen gas while being stirred with an anchor stirrer at roomtemperature for 1 hour. It was then heated to an internal temperature of58° C. using an oil bath, after which 0.2 g of Vazo 64™ (DuPont)(2,2′-azobis(isobutyronitrile)) in solution in 5 g of DMF was added.After a polymerization time of 24 hours, the batch was cooled to roomtemperature and DMF was distilled off on a rotary evaporator. Analysisby GPC (test C, PMMA standards) gave a molecular weight M_(n) of 2 820g/mol and M_(w) of 7 540 g/mol.

Subsequently this oligomeric polyacrylate, 300 g of 2-ethylhexylacrylate and 60 g of methyl acrylate were dissolved in 150 g ofacetone/n-butanol (7:3) and the solution was rendered inert usingnitrogen gas for 1 hour and then heated to an internal temperature of58° C. again. At this temperature, 0.2 g of Vazo 64™ (DuPont)(2,2′-azobis-(isobutyronitrile)) in solution in 5 g of acetone wasadded. The polymerization was conducted at a constant externaltemperature of 70° C. Following a reaction time of 6 hours, the batchwas diluted with 80 g of acetone. After a reaction time of 24 hours, afurther 0.2 g of Vazo 64™ (DuPont) (2,2′-azobis(isobutyronitrile)) insolution in 5 g of acetone was added. After 30 hours the batch wasdiluted with 50 g of acetone. The polymerization was terminated bycooling to room temperature after a reaction time of 48 hours. Analysisby GPC (test C, PMMA standards) gave a molecular weight M_(n) of 166 000g/mol and M_(w) of 421 000 g/mol.

Thereafter the solvent was removed in a drying cabinet at 60° C. under avacuum of 10 torr, a 30% strength solution in acetone was prepared, 2%by weight aluminum acetylacetonate, based on the polymer, in a 3%strength solution in acetone were admixed to the polymer, and thispolymer was applied from solution to a primed PET film 23 μm thick.After drying for 10 minutes at 120° C. in a drying cabinet, theapplication rate of the polymer was 50 g/m². The technical adhesiveproperties were tested by carrying out test methods A and B.

EXAMPLE 2

For carrying out example 2, the procedure of example 1 was repeated.Following preparation of the 30% strength solution in acetone, thepolymer was reacted with 2.5% by weight 3-(2-aminoethylamino)propylamineand this solution was applied to a primed PET film 23 μm thick. Afterdrying for 10 minutes at 120° C. in a drying cabinet, the applicationrate of the polymer was 50 g/m². The technical adhesive properties weretested by carrying out test methods A and B.

EXAMPLE 3

A 1 L glass reactor conventional for free-radical polymerizations wascharged with 500 g of butyl acrylate, 50 g of methyl acrylate, 50 g ofN-tert-butylacrylamide and 1.480 g of the difunctional alkoxyamine(XVII). Before the reaction is started, the entire mixture is cooled to−78° C. and degassed a number of times. It is then heated under pressureat 120° C. in the pressuretight vessel. The polymerization is terminatedafter 48 hours, the polymer being isolated and purified by dissolving itin dichloromethane and then precipitating it from methanol cooled at−78° C. The precipitate was filtered off on a chilled frit. The productobtained was concentrated in a vacuum drying cabinet at 45° C. and 10torr for 12 hours. Analysis by GPC (Test C, PMMA standards) gave amolecular weight M_(n) of 74 000 g/mol and M_(w) of 123 000 g/mol.

The polymer was then dissolved in THF and mixed with 2% by weight4-methyl-m-phenylene diisocyanate, based on the polymer. This solutionwas applied to a primed PET film 23 μm thick. After drying for 15minutes at 140° C. in the drying cabinet, the application rate of thepolymer was 50 g/m². The technical adhesive properties were tested bycarrying out test methods A and B.

EXAMPLE 4

The polymer was prepared by the same procedure as in example 3. Thepolymer was then dissolved in THF and reacted with 2% by weight1,2,7,8-diepoxyoctane and 0.05% by weight zinc chloride. This solutionwas applied to a primed PET film 23 μm thick. After drying for 15minutes at 140° C. in the drying cabinet, the application rate of thepolymer was 50 g/m². The technical adhesive properties were tested bycarrying out test methods A and B.

EXAMPLE 5

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 40 g of dimethylaminoethyl acrylate, 0.4 g ofbis-2,2′-phenylethyl trithiocarbonate and 100 g of acetone. The batchwas rendered inert with nitrogen gas while being stirred with an anchorstirrer at room temperature for 1 hour. It was then heated to aninternal temperature of 58° C. using an oil bath, after which 0.2 g ofVazo 64™ (DuPont) (2,2′-azobis(isobutyronitrile)) in solution in 5 g ofacetone was added. After a polymerization time of 24 hours, a further0.2 g of Vazo 64™ (DuPont) (2,2′-azobis(isobutyronitrile)) in solutionin 5 g of acetone was added. After a reaction time of 48 hours, thebatch was cooled to room temperature and acetone was distilled off on arotary evaporator. Analysis by GPC (test C, PMMA standards) gave amolecular weight M_(n) of 4 850 g/mol and M_(w) of 8 790 g/mol.

Subsequently this oligomeric polyacrylate, 300 g of 2-ethylhexylacrylate and 60 g of methyl acrylate were dissolved in 150 g of acetoneand the solution was rendered inert using nitrogen gas for 1 hour andthen heated to an internal temperature of 58° C. again. At thistemperature, 0.2 g of Vazo 64™ (DuPont) (2,2′-azobis(isobutyronitrile))in solution in 5 g of acetone was added. The polymerization wasconducted at a constant external temperature of 70° C. Following areaction time of 6 hours, the batch was diluted with 80 g of acetone.After a reaction time of 24 hours, a further 0.2 g of Vazo 64™ (DuPont)(2,2′-azobis(isobutyronitrile)) in solution in 5 g of acetone was added.After 30 hours the batch was diluted with 50 g of acetone. Thepolymerization was terminated by cooling to room temperature after areaction time of 48 hours. Analysis by GPC (test C, PMMA standards) gavea molecular weight M_(n) of 134 000 g/mol and M_(w) of 376 000 g/mol.

Thereafter the solvent was removed in a drying cabinet at 60° C. under avacuum of 10 torr, a 30% strength solution in acetone was prepared, 3%by weight adipic acid were admixed to the polymer, and this polymer wasapplied from solution to a primed PET film 23 μm thick. After drying for10 minutes at 130° C. in a drying cabinet, the application rate of thepolymer was 50 g/m². The technical adhesive properties were tested bycarrying out test methods A and B.

EXAMPLE 6

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 360 g of 2-ethylhexyl acrylate, 40 g of methyl acrylate, 40g of isobornyl acrylate, 0.6 g of S,S-di(benzyl-4-phenylaceticacid)trithiocarbonate (XX) and 150 g of acetone. The batch was renderedinert with nitrogen gas while being stirred with an anchor stirrer atroom temperature for 1 hour. It was then heated to an internaltemperature of 58° C. using an oil bath, after which 0.2 g of Vazo 64TM(DuPont) (2,2′-azobis(isobutyronitrile)) in solution in 5 g of acetonewas added. After a polymerization time of 24 hours, a further 0.2 g ofVazo 64™ (DuPont) (2,2′-azobis(isobutyronitrile)) in solution in 5 g ofacetone was added. After a reaction time of 48 hours, the batch wascooled to room temperature. Analysis by GPC (test C, PMMA standards)gave a molecular weight M_(n) of 125 200 g/mol and M_(w) of 187 500g/mol.

Thereafter this polyacrylate was diluted to a 30% strength solution inacetone and mixed with 2% by weight 4-methyl-m-phenylene diisocyanate,based on the polymer. This polymer was applied from solution to a primedPET film 23 μm thick. After drying for 15 minutes at 140° C. in a dryingcabinet, the application rate of the polymer was 50 g/m². The technicaladhesive properties were tested by carrying out test methods A and B.

EXAMPLE 7

The polymer was prepared by the same procedure as in example 8. Thepolymer from example 8 was reacted with 1.5% by weight Starburst™(PAMAM) Dendrimer Generation 2.0 (20% strength solution in methanol),the dendrimer containing 16 amino groups and having an M_(w) ofapproximately 3 256 g/mol. This solution was applied to a primed PETfilm 23 μm thick. After drying for 15 minutes at 140° C. in the dryingcabinet, the application rate of the polymer was 50 g/m². The technicaladhesive properties were tested by carrying out test methods A and B.

Results:

In examples 1 and 2 a polymer with enriched polyacrylic acid in theendblocks was prepared. GPC analysis showed that not very many units canbe located at the end of the polyacrylate block (according to M_(n)=1410 g/mol per end).

If this polymer is then reacted, then the endblocks can be crosslinkedin a targeted manner in the process of the invention. In example 1,crosslinking was carried out thermally using aluminum chelate. Besidesaluminum chelate it is also possible to carry out crosslinking withpolyfunctional amines, in which case the linking is via an ammoniumsalt.

Following examples 1 and 2, in which the endgroup-functionalizedpolyacrylates were prepared by way of the addition of monomer, inexamples 3 and 4 the functional group was attached to the polymer by wayof the regulating substrate. The polymer from example 3 and 4 wasprepared by means of a hydroxy-functionalized regulator (XVII). Thecoupling of the hydroxy endgroups in the polyacrylate then took placeultimately, in example 3, by way of a difunctional isocyanate. Forcrosslinking of the hydroxy endgroup, it is also possible, however, touse a difunctional epoxide, in a reaction which takes place preferablycatalyzed by Lewis acid. Accordingly, the coupling reaction in example 4was carried out using zinc chloride as the catalyst.

In example 5, a polyacrylate was prepared, by means of trithiocarbonateregulator, which carries a plurality of amine endgroups on both polymerchain ends. For coupling it was reacted with adipic acid. In this case,again, the coupling reaction proceeds via an acid-base reaction.

In examples 6 and 7, a functionalized regulator was used. Thetrithiocarbonate XX possesses two carboxylic acid functions, which byway of the polymerization mechanism are incorporated into the polymer atthe respective chain end. In example 6, the corresponding polymer wascoupled with a difunctional isocyanate. In example 7, the species usedfor coupling was a dendrimer carrying 16 amine functions in the outergroup. With this coupling it is possible to construct star polymers andalso to crosslink them.

The table below summarizes the technical adhesive data. The tableclearly indicates that the process of the invention is very suitableindeed for preparing pressure sensitive adhesives. SST 23° C., 10 NBS-steel Material [min] [N/cm] Reference 1 1 3.2 Example 1 955 4.4Example 2 585 4.9 Example 3 710 3.9 Example 4 915 3.6 Example 5 785 4.0Example 6 680 4.2 Example 7 470 4.4Application rate: 50 g/m²BS: Immediate bond strength to steelSST: Shear stability times

By means of the inventive crosslinking techniques chosen it is possibleto raise the cohesion of prepared pressure sensitive adhesives markedlyin comparison with the reference.

1. A process for increasing the molecular weight of polyacrylates,wherein polyacrylates functional at least on some of their chain ends byfunctional groups X are reacted with at least one compound containing atleast two functional groups Y capable of entering into linkingreactions, in the form of addition reactions, with the functional groupsX.
 2. A process for increasing the molecular weight of polyacrylates,wherein polyacrylates functional at least on some of their chain ends byfunctional groups X are reacted with at least one compound containing atleast two functional groups Y capable of entering into linkingreactions, in the form of substitution reactions, with the functionalgroups X.
 3. A process for increasing the molecular weight ofpolyacrylates, wherein polyacrylates functional at least on some oftheir chain ends by functional groups X are reacted with at least onecompound containing at least two functional groups Y capable of bondingthe polyacrylates using the functional groups X.
 4. A process as claimedin claim 1, 2 or 3, wherein the polyacrylates functionalized with thefunctional groups X have an average molecular weight (number average)M_(n) in the range from 2000 g/mol to 1 000 000 g/mol.
 5. A process asclaimed in claim 1, 2 or 3, wherein the increase in the molecular weightis achieved by a crosslinking of the polyacrylate.
 6. A process asclaimed in claim 1, 2 or 3, wherein the linking reactions proceed tolink the polyacrylates containing the functional groups X and thecompounds containing the functional groups Y linearly to one another. 7.A process as claimed in claim 1, 2 or 3, wherein the polyacrylatescontaining functional groups X contain at least one chain branch.
 8. Aprocess as claimed in claim 7, wherein the polyacrylates contain atleast three functional groups X.
 9. A pressure sensitive adhesive forsingle-sided and double-sided pressure sensitive adhesive tapescomprising a polyacrylate modified by the process of claim 1, 2 or 3.